This invention relates to an instrument for use in a joint replacement procedure.
The success of a joint replacement procedure depends in part on the appropriate alignment of the articulating parts of the joint prosthesis. Inappropriate alignment can give rise to post-operative problems, including limited range of motion of the joint, wear of the bearing surfaces which is uneven or excessive, and damage to soft tissue which is associated with the joint.
It is known to construct the humeral component of a shoulder joint prosthesis using a stem part, a head part, and a connector part which can be fitted into a bore in the head part, and has a bore formed in it to receive a spigot on the stem part. Such a device is disclosed in US-A-2005/0197708. The eccentric arrangement of the bore in the head part relative to the axis of the head part, and the eccentric arrangement of the bore in the connector part eccentric relative to the axis of the connector part, allow the distance through which the head part is offset relative to the stem part, and the orientation of that offset, to be adjusted to suit the requirements of a patient. This can facilitate variation of effective arm length, and the extent of retroversion and anteversion.
It is known to use trial implant instruments in a surgical procedure to implant an orthopaedic prosthesis. The use of a trial instrument has the advantage that the configuration of the instrument can be assessed in relation to a mating instrument or implant with a view to selecting the ultimate implant component with the optimum configuration.
US-A-2001/0053935 discloses a trialling system which comprises a plurality of components. It is suitable for use in conjunction with an implant which also comprises a plurality of modular components. The modular trial implant instrument can then enable the configuration of the trial implant instrument to be changed to identify an optimum configuration.
It is known for trialling systems to incorporate markings to allow for easy identification of the configuration of the trial implant instrument such that this information can be recorded and transferred to the final orthopaedic prosthesis. In particular, for trialling systems in which the head part can be offset relative to the stem part, markings can be provided to indicate the orientation of the head part (that is, the direction of maximum eccentricity). However, depending upon the configuration chosen for the trial implant instrument, the markings may not be easily visible to a surgeon. For instance, if a significant degree of anteversion is applied to a trial implant instrument which includes a marker indicating the radial direction of maximum eccentricity (for a predetermined magnitude of the offset), the marker may be obscured within the wound. The problem is particularly acute for minimally invasive surgery where visibility of implants and trial implant instruments in general is limited. Limited visibility can result in misidentification of the correct marking indicating the radial direction of maximum eccentricity.